1. Field of the Invention
This invention relates to an optical disc apparatus, and more particularly, to a write once type optical disc apparatus in which a laser beam is emitted onto an optical disc to sequentially form pits, thereby recording desired data.
2. Description of the Related Art
Heretofore, an optical disc apparatus has been proposed in which a light beam is emitted onto a disc type recording medium to form pits sequentially thereby recording desired information. A CD-R (CD-Recordable) drive based upon the CD (Compact Disc) Standard has been produced as this type of an optical disc apparatus.
The optical disc of this kind consists of a polycarbonate substrate (PC Substrate), a spin-coated organic pigment recording layer (Dye Recording Layer), a reflection layer of deposited gold vapor (Reflection Layer), and a plastic protective layer (Protective Layer).
As shown in FIGS. 1A to 1D, the optical disc apparatus generates such a modulated signal S1 (FIG. 1A) so that the generation probabilities of logic "0" and "1" become equal, by performing EFM (Eight to Fourteen Modulation) on a recording data. A laser diode is driven on the basis of this modulated signal S1. Therefore, in the optical disc apparatus, a light beam is emitted onto the optical disc intermittently corresponding to the logic level of the modulated signal S1, and the organic coloring matter of the data recording plane is altered by heat. Thereby, the area having the low reflection rate, that is, the pits, are formed on the data recording plane. In addition, the light beam emitted from a laser is high for recording.
The optical disc apparatus generates the modulated signal S1 so that H level and L level continue within a cycle 3T to 11T based on the predetermined reference cycle T. Thus, the optical disc apparatus records the desired data by sequentially forming pits P on the data recording plane (FIG. 1B). In addition, the portions where pits P are not formed are called "lands".
In reproducing, the light beam of low output is emitted for reproducing, and receives the reflected light from the optical disc at a photo detector. Thus, the optical disc apparatus obtains the reproduced signal RF the level of which changes depending on the intensity of the reflected light (FIG. 1C). Then, the signal level of the reproduced signal RF is detected based on the predetermined slice level SL, thereby detecting the reproduced data D1 (FIG. 1D).
At this time, since the recording signal S1 consists of such recording signals that the generation probabilities of the logic "0" and "1" by EFM are equal, the optical disc apparatus selects and sets the slice level SL so that the generation probabilities of logic "0" and "1" become equal even in the reproduced data D1. Thus, the bit error rate is reduced.
On the contrary, in recording, since the organic coloring matter are heat-altered to form pits, the size of the pits change depending upon the surrounding temperature, the sensitivity of the organic coloring matter, and the wave length of the laser, even when the laser diode is driven under the same conditions. Therefore, the optical disc system changes the driving power of the laser diode one after another and records the test data in the predetermined test writing area in the optical disc, and thereafter reproduces the test data to detect a plurality of asymmetry values Asy. Then, the optical disc apparatus selects the asymmetry value Asy, which is the closest value to the predetermined asymmetry value, among the detected asymmetry values, and selects the driving power at the time as an optimum value. The laser diode is driven by the selected driving power and the desired data is recorded. Therefore, the optical disc apparatus constantly forms pits having fixed size and reduces jitter of the reproduced signal, etc.
Here, the asymmetry value signifies the ratio of time means of pits and lands, and is expressed by the relation formula, using the slice level SL at which the generation probabilities of the logic "0" and "1" become equal in the reproduced data D1, a peak level TOP of the reproduced signal, and a bottom level BTM of the reproduced signal.
For this reason, the optical disc apparatus, in test writing, the asymmetry value Asy is simply detected by using an asymmetry detecting circuit 1 shown in FIG. 2.
That is, the optical disc apparatus reproduces the test data recorded in the test writing area, and inputs the reproduced signal RF obtained as the result to the asymmetry detecting circuit 1.
The reproduced signal RF is supplied to a clamp circuit 3, after the direct current component is removed at an input capacitor 2. Thereby, as shown in FIG. 3, the bottom level of the reproduced signal RF is held to "0" level. A comparator circuit 4 compares the slice level SL with the output signal of the clamp circuit 3 and supplies inverting output and non-inverting output, which are the compared result, to low pass filter circuits 5 and 6. The low pass filter circuits 5 and 6 convert the output of the comparator circuit 4 into direct current signals respectively, to supply to a differential amplifier 7.
The differential amplifier 7 inputs the output signal of the low pass filter circuits 5 and 6 to amplify the differential components. The output signal of the differential amplifier 7 is fed back to the input terminal of the comparator circuit 4 as the slice level SL.
In this way, the asymmetry detecting circuit 1 sets the slice level SL so that the generation probabilities of the logic "0" and "1" become equal.
A peak hold circuit 8 peak holds the output signal of the clamp circuit 3 and detects the signal level (A+B) between the bottom level BTM and the peak level TOP of the reproduced signal RF.
The output signal and the slice level SL of the peak hold circuit 8 are converted into digital values at analog/digital converting circuits (A/D) 9 and 10 to supply to a system control circuit (not shown). The system control circuit performs the calculating process which is indicated by the formula: ##EQU1## on each driving power, and detects the asymmetry value Asy.
Then, the system control circuit selects the asymmetry value, which is the closest value to the predetermined asymmetry value, among the detected asymmetry values, and selects the driving power of the laser at the time when the selected asymmetry value is obtained as the optimum value. Then, the following data are written with the selected driving power of the laser. Therefore, since the laser driving power in writing is set to the optimum value, the bit error rate is improved by reducing the jitter in reproducing.
The test writing area is formed in the inner side of the optical disc, and the size of test writing area is limited to a specific size. Here, according to the conventional system, the slice level SL is obtained by using the low pass filter circuit.
Hence, it is necessary to enlarge the time constant of the low pass filter circuit to some size, and the response is slow, thereby taking much time until the slice level is stable to a fixed DC level. Therefore, the time to perform a test writing by each laser power becomes long, and much test writing area is used as a result. Here, because this optical disc is the write once type, the area in which a test writing is performed once can not be used for a second test writing. Therefore, if it takes much time to perform one test writing, the number of times to perform a test writing is reduced. That is, this means that the number of times to write a desired data reduces. To solve this problem, it can be considered to reduce the number of times to change the laser power in one test writing, but this needs to select the optimum asymmetry value from the detected result of few asymmetry values. That is, there is a problem that the optimum asymmetry value may not be detected and the writing is performed by the laser power determined by non-optimum asymmetry.